PROJECT SUMMARY Abdominal aortic aneurysm (AAA) disease is a frequent cause of morbidity and mortality. Roughly 25,000 AAA repairs are performed each year, and AAAs account for over 13,000 deaths annually in the United States. The underlying mechanisms of AAA formation are unknown, which has hampered development of effective medical therapies. Here, we focus on the novel hypothesis that myeloperoxidase (MPO), a leukocyte enzyme expressed primarily in neutrophils (PMNs) that utilizes H2O2 to promote oxidative stress and inflammation, plays a key role in the pathogenesis of AAA. We provide novel preliminary data showing that MPO accrues in the aorta during AAA formation, and that genetic deletion of Nox4, a unique high-output enzymatic producer of H2O2 that is highly expressed in smooth muscle cells (SMCs), blocks MPO uptake and prevents AAA. We further propose that PMN activation/MPO accrual in AAA are regulated by prostaglandin D2 (PGD2) and nicotinic acid (GPR109A) receptors, and that ?repurposing? available drugs that target these inflammatory cell receptors is an attractive therapeutic strategy in AAA. Our central hypothesis is that MPO cooperates with Nox4 to promote deleterious oxidative stress in AAA, which can be therapeutically modulated by targeting PGD2 receptors and GPR109A. Three aims are proposed to test our central hypothesis: in Aim1, we will test the hypothesis that Nox4 cooperates with MPO in the pathogenesis of AAA using genetically modified mice and a newly developed mass spectrometry assay that will enable us to determine whether aortic MPO uptake and activity are dependent upon Nox4 during AAA formation. We will also determine whether overexpression of human MPO in hematopoietic cells enhances AAA formation, and if so, whether it can be overcome by SMC Nox4 deletion. In Aim 2, we will test the hypothesis that PGD2 receptors promote PMN/MPO recruitment and MPO-mediated AAA formation using genetically modified mice in which one or both alleles of DP1 or DP2 have been deleted, coupled with and pharmacologic DP receptor agonists and antagonists. In Aim 3, we will test the hypothesis that GPR109A modulates PMN activation and MPO-mediated AAA formation using GPR109A knockout mice coupled with pharmacologic approaches. A particular emphasis of Aims 2 and 3 is to identify promising therapeutic strategies that are both feasible and effective for patients with AAA. This application addresses two significant gaps in the field: i) identifying fundamental mechanisms of AAA formation and ii) using this knowledge to advance medical treatment for patients with AAA.